Javascript. Global variable isnt recognized by while loop

Its because that your code goes into the infinite loop.

Since in your code random number is generated only once. and there is possibility that currentCard !== 'Spade' becomes true always, and it run into infinite loop/

The best way to check it: use browser console.

ctrl(cmd)+shift+i 

End just paste your code and check it

The second sample code you provide has a problem due to the fact that the randomNumber variable is assigned only once. The variable is passed by value to your loop, not by reference, meaning it will not change while inside the loop.

This means that any value assigned to your randomNumber variable will remain the same until changed somewhere else in your code. Consider the following examples:

Example 1

var cards = ['Diamond', 'Spade', 'Heart', 'Club'];
var currentCard = 'Heart';
var randomNumber = Math.floor(Math.random() * 4);

while (currentCard !== 'Spade') {
  console.log(currentCard);
  randomNumber = Math.floor(Math.random() * 4);
  currentCard = cards[randomNumber];
}

console.log('Found a Spade!');

In this case, the variable is global, but its value changes in every iteration, just like it does in the tutorial you are following. The only difference is the scope of the variable.

Example 2

var cards = ['Diamond', 'Spade', 'Heart', 'Club'];
var currentCard = 'Heart';

while (currentCard !== 'Spade') {
  console.log(currentCard);
  currentCard = cards[Math.floor(Math.random() * 4)];
}

console.log('Found a Spade!');

In this case you do not use a variable at all, just the result of the functon. This is not the kind of solution you were trying to implement, it just serves to show you how you can use a function's result and change it at every iteration of your loop.

To sum it all up:

The randomNumber variable does not keep a reference to the function called, so its value remains unchanged no matter how many times you access it in the loop. If you want it to be global, you can change its value inside the loop, as shown in Example 1. You can use the output of the function as-is, without assignment to a variable, using the method shown in Example 2.